Conventionally, a head-mounted display (hereinafter, referred to as an “HMD”) has been known as a display device that is mounted on a head of a user. A method in which an image is projected on a part of the user's field of view has been proposed as a display method of the HMD. In the HMD of such a display method, it is desirable that a position within the field of view an image is to be projected is freely adjustable by the user. A shape of the head and positions of eyes of the user are different from each other for each user. Thus, it is desirable that the HMD is allowed to be adjusted about a position within the field of view an image is to be projected with precision and a high degree of freedom. In contrast thereto, there is a known head-mounted display device that is capable of adjusting a position of a display unit by using two coupling mechanisms each having 3 degrees of freedom.